The present invention concerns implantable cardiac pacemakers, more specifically signal processing methods and apparatus for detecting intracardiac signals for controlling pacing operation and for monitoring the intracardiac electrocardiogram (ECG) externally.
Most cardiac pacemakers function according to the pacing mode that is inhibited: a stimulation pulse is delivered only if no spontaneous activity is detected during a period of sensing corresponding to the duration of a cardiac cycle (800 ms for a normal rhythm of 75 cpm). This mode of functioning allows an economy of energy, that is to say an increase of the longevity of the pacemaker.
An inhibited pacing mode also allows, from the physiological point of view, to avoid a competition between the spontaneous rhythm of the patient and the frequency of stimulation of the cardiac pacemaker. Competition is not avoided in an asynchronous pacing mode.
The cardiac activity, represented by a variation of cell potential of the myocardium, is sensed in a cardiac pacemaker by means of electrodes placed on the wall of the atrium and/or the ventricle, according to the type of the cardiac pacemaker (AAI, VVI, DDD).
Some problems linked to the sensing are due to the perturbation potentials created by the pacemaker itself. Indeed, stimulation pulses provoke important variations of the cell potential, in the stimulated cardiac chamber as well as in the non stimulated chamber. The sensing of a stimulation pulse in the cardiac chamber that is not stimulated is a particular problem during the ventricular sensing period following an atrial stimulation. Indeed, it is necessary to sense the ventricle as soon as possible after the stimulation so as to verify the good atrial-ventricular conduction.